1. Field of the Invention
This invention relates to an improved process for producing calcium cellulose glycolate. This compound is a tasteless, odorless cellulose derivative which is used industrially as a disintergrating agent for tablets for use in medicines and foods. Calcium cellulose glycolate is compression moldable and water insoluble. It swells upon the addition of water to disintegrate.
2. Description of the Prior Art
A process for making calcium cellulose glycolate is described in Japanese Patent Publication No. 7960/1968 (hereinafter "the Japanese application"). In that process, purified cellulose glycolic acid is neutralized with calcium carbonate in the presence of water, dried and then pulverized. The use of calcium carbonate is the most significant feature of the process. Its use results in the production of gaseous carbon dioxide as a by-product which eliminates the need for the purification of the reaction product and results in the production of a product of high purity. In addition, the use of calcium hydroxide, calcium acetate, calcium chloride and calcium phosphate are listed as calcium-substituting agents and compared.
The purified cellulose glycolic acid used as the starting material for the process of the Japanese application can be obtained by adding a strong acid such as sulfuric acid or hydrochloric acid to sodium cellulose glycolate and then eliminating the resulting salt as a by-product.
Sodium cellulose glycolate can be prepared by reacting pulp with sodium hydroxide and mono-chloro acetic acid. There are two principle methods known in the art. One method employs water as the reduction medium. The other method employs a water lower alkyl alcohol mixture as the reaction medium. The former will be hereinafter referred to as the aqueous medium method and the latter will be referred to as the solvent medium method.
When carrying out the production of sodium cellulose glycolate by the solvent medium method, the reaction of the mono-chloro acetic acid is more efficient and the product that is obtained has a higher degree of substitution as compared with the production of the sodium cellulose glycolate by the aqueous medium method. For these reasons, the solvent method is considered the more advantageous process for producing sodium cellulose glycolate. However, the solvent method has not been of any use in the production of calcium cellulose glycolate, since it does not produce cellulose glycolic acid. Thus, as discussed above, the Japanese application describes a method for producing calcium cellulose glycolate from cellulose glycolic acid. If the solvent method did produce cellulose glycolic acid, at some point in the process, that process might have been useful in the production of calcium cellulose glycolate.
In contrast, in the aqueous medium method, the sodium cellulose glycolate which is prepared is converted into the free acid, cellulose glycolic acid, in the aqueous medium. The resulting cellulose glycolic acid is precipitated and washed with water for purification. Thus, cellulose glycolic acid at a high purity can be obtained in the purification step. Accordingly, the ability to use the cellulose glycolic acid as the starting material for the process of the Japanese application is an advantage of the aqueous medium method over the solvent method. In fact, the cellulose glycolic acid used as the starting material in the industrial production of calcium cellulose glycolate is obtained by the aqueous medium method described above.
The aqueous medium method, however, does have disadvantages. For example, the effective rate of utilizing the etherifying agent upon etherification is lower as compared with the solvent method. Thus, highly etherified product is difficult to obtained by the aqueous medium method.